Two distinct phases of continental magmatism are evident in western Arabia. The first, from about 30 to 20 Ma, produced tholeiitic-to-transitional lavas emplaced along NW trends. The second, from about 12 Ma to Recent, produced transitional-to-strongly-alkalic lavas emplaced along N-S trends. The older phase is attributed to passive-mantle upwelling during extension of the Red Sea Basin, whereas the younger phase is attributed to active-mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision. The younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the West Arabian Swell after about 14 Ma. A variety of evidence suggests that the West Arabian Swell is thermally supported by hot, upwelling asthenosphere. In contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the Red Sea Basin, these processes were symmetric across a N-S line marking the central axis of the West Arabian Swell. This axis coincides with two fundamental features: the Ha'il-Rutbah Areh in the north, and the Makkah-Madinah-Nafud (MMN) volcanic line in the south. The symmetry of magmatism is demonstrated by petrochemical evidence that the MMN harrats were derived by greater degrees of partial melting, at shallower depths, than those harrats lying to the west and east of the MMN line. The potential temperature of the asthenosphere is estimated to be about 1436 ¿C beneath the MMN line and about 1354 ¿C beneath the flanking, more undersaturated harrats.

The source of upwelling is either a mantle plume centrally located beneath the West Arabian Swell or an elongated and extended lobe of hot asthenosphere emanating from the Ethiopian mantle plume. Convective flow may have been channelled along a preexisting, regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the Afro-Arabian Dome. This crest of mantle upwelling underlies the MMN line in the north and the Danakil Depression and Ethiopian rift system in the south. It also passes through the Red Sea Basin at the midpoint of an unusual, doubly propagating rift system where axial seafloor spreading began 4--5 m.y. ago. The NW trend of the Red Sea Basin was well established by crustal attenuation during the older magmatic phase. The Pliocene invasion of this basin by a N-S zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions. ¿ American Geophysical Union 1992